1. Field of the Invention:
The invention relates to the field of magnetic bubbles and particularly to detectors of magnetic bubbles.
2. Prior Art:
In single wall domain (magnetic bubble) devices, such as bubble memories, detectors are used to sense magnetic bubbles. Most frequently, a magnetic bubble is expanded and then propagated through a magnetoresistive element. The resistance change of this element caused by the expanded bubble is detected.
Often bubble detectors include a detector element and a dummy element. The signal from the dummy element is used as a reference in a bridge network. The effects of the rotating magnetic field on these two elements ideally provides a null or zero signal with no magnetic bubble at the detector element, and a signal (e.g. 6 mvolts) when a bubble is at the detector element. One arrangement where the output of two detectors is used in a single Wheatstone bridge is described in "Megabit Bubble-Memory Chip Gets Support from LSI Family" Electronics, Apr. 26, 1979, beginning on p. 105. Also see copending application, Ser. No. 65,361 filed Aug. 8, 1979 entitled "MAGNETIC BUBBLE DETECTOR" assigned to the assignee of the present invention.
One problem which has consistently plagued fabricators of magnetic bubbles is that the signal output of the magnetoresistive elements varies from wafer-to-wafer because of process variations. In some instances instead of a "zero" signal level with no bubble present the detector output is, for example, 2 mV signal level. The presence of this signal interferes with the reliable detection of bubbles.
To overcome this problem, a number of steps have been taken. In some instances, if the detector output is not within predetermined limits, the memory is not used. Obviously, this reduces yields. In other instances, sense amplifiers which are more tolerant to signal shape or level variations are used. This circuitry is more costly and not ideal for production memories. The matching of particular devices with particular sense amplifiers is also used. This method has the obvious objection of being time consuming and is not a viable manufacturing technique.
As will be seen, the present invention provides a method for adjusting the output from a bubble detector to provide a true null or zero level, or any desired signal level. The method lends itself to high production fabrication.